With the development of digital technologies, there is an increasing trend of high-speed processing or preservation of a large capacity data. Because of this, large-scale integration and high performance are required in semiconductor devices for use in electronic equipments.
Accordingly, to realize large-scale integration of a semiconductor memory device (DRAM), extensive studies and development are carried out on the techniques using a ferroelectric material or a high-permittivity material, in place of a silicon oxide or a silicon nitride, as a capacitor insulating film for a capacitance element constituting the DRAM.
Also, to realize a nonvolatile RAM capable of performing high-speed write/read operation at a low voltage, studies and development are being conducted actively in regard to a ferroelectric memory (FeRAM) using a ferroelectric film having a spontaneous polarization characteristic, as a capacitor insulating film.
The ferroelectric memory (FeRAM) stores information utilizing a hysteresis characteristic of a ferroelectric substance. The ferroelectric memory includes in each memory cell a ferroelectric capacitor having a ferroelectric film, as a capacitor dielectric film disposed between a pair of electrodes. In the ferroelectric substance, polarization occurs depending on a voltage applied between the electrodes, and spontaneous polarization remains even after the applied voltage is removed. Also, when the polarity of the applied voltage is reversed, the polarity of the spontaneous polarization is also reversed. Therefore, information can be read out by detecting the spontaneous polarization. Further, as compared with a flash memory, the ferroelectric memory is operable at a low voltage, and capable of high-speed writing with reduced power.
The ferroelectric film of the ferroelectric capacitor is formed of a PZT-based material, such as lead zirconate titanate (PZT) and La-doped PZT (PLZT), a compound having a bismuth layer structured ferroelectrics, such as SrBi2Ta2O9 (SBT,Y1) and SrBi2(Ta,Nb) 2O9 (SBTN,YZ) Conventionally, as a film-forming method of the ferroelectric film, there has been used a sol-gel method, a sputtering method or an MOCVD method. By means of such the film-forming method, a ferroelectric film of an amorphous phase is formed on a bottom electrode film, and thereafter, the ferroelectric film is crystallized to a crystal of perovskite structure by a thermal treatment. A crystallinity of the ferroelectric film is subject to a crystallinity of the bottom electrode film, the film-forming condition and the crystallization condition of the ferroelectric film, and the like. Therefore, conventionally, by adjusting such the conditions, it has been intended to improve the crystallinity of the ferroelectric film. However, in recent years, it has become unable to satisfy the requirements of further improvement on the crystallinity. As a result, a ferroelectric capacitor having sufficient characteristics cannot be obtained any more, nor variation in the performance of the memory cell in an identical chip can be suppressed sufficiently.
Also, for example, in Patent document 1 (Japanese Patent Application Laid-open No. 2003-2647), there is disclosed the adoption of a variety of compositions of ferroelectric films for the purpose of decreasing a crystallization temperature of the ferroelectric film. However, there is no such method that can obtain sufficient crystallinity.
Patent document 1: Japanese Patent Application Laid-open No. 2003-2647